Constraint of Dithering of Source Actuations

ABSTRACT

A system for constraining a dither time can comprise a source and a controller coupled to the source. The controller can be configured to actuate the source in sequence, while the source is moving through a fluid volume at a bottom speed, with an actuation time interval between each actuation comprising a sum of a nominal time and a dither time for each actuation and constrain the dither time for each actuation such that a reduction of the actuation time interval relative to a directly precedent actuation time interval is at most a threshold dither time difference, wherein the threshold dither time difference corresponds to a maximum bottom speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application62/376,048, filed Aug. 17, 2016, which is incorporated by reference.

BACKGROUND

In the past few decades, the petroleum industry has invested heavily inthe development of marine seismic survey techniques that yield knowledgeof subterranean formations beneath a body of water in order to find andextract valuable mineral resources, such as oil. High-resolution imagesof a subterranean formation are helpful for quantitative interpretationand improved reservoir monitoring. For a typical marine survey, a marinesurvey vessel tows one or more sources below the water surface and overa subterranean formation to be surveyed for mineral deposits. Receiversmay be located on or near the seafloor, on one or more streamers towedby the marine survey vessel, or on one or more streamers towed byanother vessel. The marine survey vessel typically contains marinesurvey equipment, such as navigation control, source control, receivercontrol, and recording equipment. The source control may cause the oneor more sources, which can be air guns, marine vibrators, etc., toproduce signals at selected times. Each signal is essentially a wavecalled a wavefield that travels down through the water and into thesubterranean formation. At each interface between different types ofrock, a portion of the wavefield may be refracted, and another portionmay be reflected, which may include some scattering, back toward thebody of water to propagate toward the water surface. The receiversthereby measure a wavefield that was initiated by the actuation of thesource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an elevation or xz-plane view of marine seismicsurveying in which acoustic signals are emitted by a source forrecording by receivers.

FIG. 2 illustrates an approach to dithering actuations of a source.

FIG. 3 illustrates constraining a dither time difference to a thresholddither time difference that is less than a maximum dither timedifference.

FIG. 4 illustrates a graph relating minimum actuation time intervals,maximum bottom speeds, and dither time differences.

FIG. 5 illustrates a method flow diagram for constraining dithering ofsource actuations.

DETAILED DESCRIPTION

This disclosure is related generally to the field of marine surveying.For example, this disclosure may have applications in marine surveying,in which one or more sources are used to generate wavefields, andsensors (towed or ocean bottom) receive energy generated by the sourcesand affected by the interaction with a subsurface formation. The sensorsthereby collect marine survey data, which can be useful in the discoveryor extraction of hydrocarbons from subsurface formations.

In a marine survey, a source may be activated in a sequence with a delaybetween actuations of the source, hereinafter referred to as anactuation time interval. A source may be activated in a sequence with adistance between actuations of the source, hereinafter referred to as anactuation distance. The actuation time interval or the actuationdistance can be short such that a signal corresponding to a firstactuation of the source overlaps a signal corresponding to a secondactuation of the source. Alternatively, at least two sources may beactuated with a short actuation time interval or a short actuationdistance such that a signal corresponding to the actuation of a firstsource overlaps a signal corresponding to the actuation of a secondsource. The actuation time interval can comprise a nominal time and adither time. As used herein, “nominal time” refers to an amount of timebetween consecutive nominal actuation points. As used herein, “nominalactuation points” refer to predetermined points on a survey path sailedby a marine survey vessel at which a source may be actuated. Nominalactuation points can be spaced apart by a predefined nominal time. Theactuation distance can comprise a nominal distance and a ditherdistance. The nominal actuation points can be spaced apart by apredefined nominal distance. As used herein, “nominal distance” refersto a distance between consecutive nominal actuation points. For example,if the nominal time is predefined as five seconds, then the nominaldistance would be the distance traveled by a source at a bottom speedduring the five seconds between consecutive nominal actuation points. Asused herein, “bottom speed” refers to the speed of a source relative tothe seafloor. Conversely, if the nominal distance is predefined as 12.5meters, then the nominal time would be the amount of time required for asource to travel the 12.5 meters between consecutive nominal actuationpoints at a predetermined bottom speed.

Actuations can be dithered to improve the separation process of a signalcorresponding to a first actuation from a signal corresponding to asecond actuation where the two signals overlap. Actuations can bedithered temporally or spatially. Actuations can be dithered temporallyby using a dither time to distinguish the signal corresponding to thefirst actuation from the signal corresponding to the second actuation.As used herein, “dither time” refers to a randomized amount of time thatcan be added to the nominal time. Actuations can be dithered spatiallyby using a dither distance to distinguish the signal corresponding tothe first actuation from the signal corresponding to the secondactuation. As used herein, “dither distance” refers to a randomizeddistance that can be added to the nominal distance. The dither time canbe generated via a randomization scheme such that the randomized amountof time is pseudorandom. Embodiments herein are not limited to anyparticular randomization scheme. A positive dither time will cause theactuation time interval to be greater than the nominal time whereas anegative dither time will cause the actuation time interval to be lessthan the nominal time. The dither time can be between a maximum negativedither time and a maximum positive dither time. Similarly, a positivedither distance will cause the actuation distance to be greater than thenominal distance whereas a negative dither distance will cause theactuation distance to be less than the nominal distance. The ditherdistance can be between a maximum negative dither distance and a maximumpositive dither distance. As a result of dithering actuations, coherencymeasures in the proper domains, or other techniques, can be utilized toactively separate the recorded data over the individual sources. Thedither time or the dither distance can be used in a deblending processto distinguish the signal corresponding to the first actuation from thesignal corresponding to the second actuation.

A problem with dithering actuations of a source may be that the distancebetween consecutive actuations at actuation points as well as the timebetween the consecutive actuations at the actuation points may beaffected by the dither times. As used herein, “actuation points” referto points at which a source is actuated rather than points at which asource may be actuated. Thus, in order to maintain a minimum actuationtime interval between consecutive actuations, the bottom speed at whicha source moves through a fluid volume may have to change during themarine survey to accommodate the varying distances and times between thepairs of consecutive actuations associated with dithering theactuations. However, by constraining a dither time difference to athreshold dither time difference that is less than a maximum dither timedifference, a faster bottom speed can be maintained during a survey forany dither time difference up to and including the threshold dither timedifference. As used herein, “dither time difference” refers to adifference between a dither time corresponding to an actuation and adither time corresponding to a directly precedent actuation. At leastone embodiment in accordance with the present disclosure includesconstraining a difference between a first dither time corresponding to afirst actuation of a source and a second dither time corresponding to asecond actuation of a source, where the first actuation is directlyprecedent to the second actuation.

It is to be understood the present disclosure is not limited toparticular devices or methods, which may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used herein, the singular forms “a”, “an”, and “the”include singular and plural referents unless the content clearlydictates otherwise. Furthermore, the word “may” is used throughout thisapplication in a permissive sense (having the potential to, being ableto), not in a mandatory sense (must). The term “include,” andderivations thereof, mean “including, but not limited to.” The term“coupled” means directly or indirectly connected.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, 230-1 mayreference element “30-1” in FIG. 2A, and a similar element may bereferenced as 330-1 in FIG. 3A. As will be appreciated, elements shownin the various embodiments herein can be added, exchanged, or eliminatedso as to provide a number of additional embodiments of the presentdisclosure. In addition, as will be appreciated, the proportion and therelative scale of the elements provided in the figures are intended toillustrate certain embodiments of the present invention, and should notbe taken in a limiting sense.

FIG. 1 illustrates an elevation or xz-plane 129 view of marine seismicsurveying in which acoustic signals are emitted by a source 126 forrecording by receivers 122. Processing and analysis of the data can beperformed in order to help characterize the structures and distributionsof features and materials underlying the solid surface of the earth.FIG. 1 illustrates a domain volume 102 of the earth's surface comprisinga solid volume 106 of sediment and rock below the solid surface 104 ofthe earth that, in turn, underlies a fluid volume 108 of water having awater surface 109 such as in an ocean, an inlet or bay, or a largefreshwater lake. The domain volume 102 shown in FIG. 1 represents anexample experimental domain for marine seismic surveys. FIG. 1illustrates a first sediment layer 110, an uplifted rock layer 112, asecond, underlying rock layer 114, and hydrocarbon-saturated layer 116.One or more elements of the solid volume 106, such as the first sedimentlayer 110 and the first uplifted rock layer 112, can be an overburdenfor the hydrocarbon-saturated layer 116. In some instances, theoverburden may include salt.

In the example of FIG. 1, the marine survey vessel 118 is a marineseismic survey vessel equipped to carry out marine seismic surveys.However, the marine survey vessel 118 can be a marine electromagnetic(EM) survey vessel equipped to carry out marine EM surveys. The marinesurvey vessel 118 can tow the source 126 such that the source 126 movesthrough the fluid volume 108 at a bottom speed. The marine survey vessel118 can tow one or more streamers 120 (shown as one streamer for ease ofillustration) generally located below the water surface 109. Thestreamers 120 can be long cables containing power and data-transmissionlines (electrical, optical fiber, etc.) to which receivers may beconnected. A controller 100 can be onboard the marine survey vessel 118and coupled to the source. The controller 100 can be configured toactuate the source 126 in sequence with an actuation time intervalbetween each actuation and constrain the dither time for each actuationsuch that a reduction of the actuation time interval relative to adirectly precedent actuation time interval is at most a threshold dithertime difference. The actuation time interval can comprise a sum of anominal time and a dither time for each actuation. The threshold dithertime difference can correspond to a maximum bottom speed. The sequencecan be an actuation sequence such that there is an actuation timeinterval between the actuations as described above. The controller 100can be configured to actuate the source 126 as the source 126 movesthrough the fluid volume 108 at a bottom speed. The bottom speed cancorrespond to the speed of the marine survey vessel 118 taking intoaccount adjustments for currents and waves.

Where the source 126 is a source array comprising more than one source,the controller 100 can be configured to actuate a first source of thesource array with a first dither time and, directly subsequent to theactuation of the first source, actuate a second source of the sourcearray after the actuation time interval comprising the nominal time anda second dither time. The controller 100 can be configured to constraina difference between the first dither time and the second dither time toat most the threshold dither time difference. The difference between thedither times associated with two consecutive actuations (a secondactuation directly subsequent to a first actuation) can be constrainedto at most a threshold dither time difference, irrespective of theconsecutive actuations being actuations of a single source orconsecutive actuations of more than one source (a first source and thena second source). The sources of a source array can be towed by a singlemarine survey vessel 118. The sources of a source array can be towed bymore than one marine survey vessel 118. For example, each of the sourcescan be towed by a different marine survey vessel 118. In at least oneembodiment, a first subset of the sources of a source array can be towedby a first marine survey vessel 118 and a second subset of the sourcesof the source array can be towed by a second marine survey vessel 118.

The source 126 can be actuated with a short actuation time interval or ashort actuation distance in between consecutive actuations of the source126. If the actuation time interval or the actuation distance is shortenough, a signal from an actuation of the source 126 may overlap, orblend with, a signal from a directly subsequent actuation of the source126. That is, the actuation of the source 126 may produce a firstreflected wavefield that is recorded by the receivers 122 on thestreamers 120 at the same time that a second reflected wavefieldproduced by the directly subsequent actuation of the source 126 isrecorded by the receivers 122 on the streamers 120. However, thereceivers 122 may not identify the reflected wavefields as correspondingto a particular actuation. By dithering the actuations, it can bepossible, for example, in post-processing, to use the dither time or thedither distance corresponding to each actuation to associate a recordedreflected wavefield with the actuation of the source that produced it. Adeblending engine can be configured to distinguish marine survey datacorresponding to a first actuation of the source 126 from marine surveydata corresponding to a second actuation of the source 126 via a dithertime for each actuation.

The streamers 120 and the marine survey vessel 118 can includesophisticated sensing electronics and data-processing facilities thatallow receiver readings to be correlated with absolute positions on thewater surface and absolute three-dimensional positions with respect to athree-dimensional coordinate system. In FIG. 1, the receivers along thestreamers are shown to lie below the water surface 109, with thereceiver positions correlated with overlying surface positions, such asa surface position 124 correlated with the position of receiver 122. Themarine survey vessel 118 can also tow one or more sources 126,comprising a source array, that produce acoustic signals as the marineseismic survey vessel 118 and streamers 120 move across the watersurface 109. Sources 126, streamers 120, or sources 126 and streamers120 may also be towed by other vessels, or may be otherwise disposed influid volume 108. For example, receivers may be located on ocean bottomcables or nodes fixed at or near the solid surface 104, and sources 126may also be disposed in a nearly-fixed or fixed configuration. For thesake of efficiency, illustrations and descriptions herein show seismicreceivers located on streamers, but it should be understood thatreferences to seismic receivers located on a “streamer” or “cable”should be read to refer equally to seismic receivers located on at leastone of a towed streamer, an ocean bottom receiver cable, or an array ofnodes. The receivers can be configured to receive marine survey datafrom consecutive actuations of the source.

FIG. 1 illustrates an expanding, spherical acoustic signal, illustratedas semicircles of increasing radius centered at the source 126,representing a down-going wavefield 128, following an acoustic signalemitted by the source 126. The down-going wavefield 128 is, in effect,shown in a vertical plane cross section in FIG. 1. The outward anddownward expanding down-going wavefield 128 may eventually reach thesolid surface 104, at which point the outward and downward expandingdown-going wavefield 128 may partially scatter, may partially reflectback toward the streamers 120, and may partially refract downward intothe solid volume 106, becoming elastic signals within the solid volume106.

FIG. 2 illustrates an approach to dithering actuations of a source. Thesource can be towed by the marine seismic vessel 218 at a bottom speed.The source, such as the source 126 illustrated in FIG. 1, may or may notbe actuated at the nominal actuation points depending on the ditheringof an actuation corresponding to an actuation point. For example, if adither time corresponding to the nominal actuation point 230-1 is zeroseconds, then the actuation point 232-1 will be the correspondingnominal actuation point 230-1. In contrast, if a dither timecorresponding to the nominal actuation point 230-1 is positive, then theactuation point 232-1 will be past the nominal actuation point 230-1 orif a dither time corresponding to the nominal actuation point 230-1 isnegative, then the actuation point 232-1 will be before the nominalactuation point 230-1. Consecutive actuations of a source can beseparated by an actuation time interval. A minimum actuation timeinterval can correspond to a period of clean recording during which asignal corresponding to an actuation of a source does not overlap asignal corresponding to a subsequent actuation of the source. A dithertime can be described as a dither distance by multiplying the dithertime by the bottom speed.

The first dither distance 234 corresponds to the nominal actuation point230-1 and a second dither distance 236 corresponds to the nominalactuation point 230-1. In the example of FIG. 2, the actuations aretemporally dithered such that the first dither distance 234 and thesecond dither distance 236 are determined from dither times. That is,the dither time can correspond to a distance offset from the nominalactuation points 230-1 and 230-2. However, embodiments are not solimited and actuations can be spatially dithered such that dither timescan be determined from the dither distances. An actuation time intervalcan be determined by subtracting the dither time difference from thenominal time. An actuation distance 240 between actuations can bedetermined from the actuation time interval by multiplying the actuationtime interval by the bottom speed. The actuation distance 240 caninclude a first portion corresponding to clean recording during which asignal corresponding to an actuation of a source does not overlap asignal corresponding to a subsequent actuation of the source and asecond portion during which the signal corresponding to the actuation ofthe source overlaps the signal corresponding to the subsequent actuationof the source.

A velocity can be described as a distance traveled divided by a time inwhich the distance is traveled. Thus, a time in which a distance istraveled can be described as the distance traveled divided by avelocity. For example, given a minimum actuation time interval(T_(MIN)), a nominal distance (D_(N)) 238, and a dither time difference(ΔT_(D)), a maximum bottom speed (S_(MAX)) can be determined. S_(MAX)refers to a maximum bottom speed at which the source can travel suchthat there is at least the minimum actuation time interval (T_(MIN))between consecutive actuations at the actuation points 232-1 and 232-2for the dither time difference (ΔT_(D)). The relationship between theminimum actuation time interval (T_(MIN)), the nominal distance (D_(N))238, the dither time difference (ΔT_(D)), and the maximum bottom speed(S_(MAX)) can be described as follows:

$\begin{matrix}{T_{MIN} = \frac{D_{N} - {S_{MAX}\Delta \; T_{D}}}{S_{MAX}}} & (1)\end{matrix}$

Solving Expression (1) for S_(MAX) yields:

$\begin{matrix}{S_{MAX} = \frac{D_{N}}{T_{MIN} + {\Delta \; T_{D}}}} & (2)\end{matrix}$

Similarly, given a minimum actuation time interval (T_(MIN)), a nominaldistance (D_(N)) 238, and a dither distance difference (ΔD_(D)), amaximum bottom speed (S_(MAX)) can be determined. As used herein,“dither distance difference” refers to a difference between a ditherdistance corresponding to an actuation and a dither distancecorresponding to a directly precedent actuation. The dither distancedifference (ΔD_(D)) can be the first dither distance 234 minus thesecond dither distance 236. S_(MAX) refers to a maximum bottom speed atwhich the source can travel such that there is at least the minimumactuation time interval (T_(MIN)) between consecutive actuations at theactuation points 232-1 and 232-2 for the dither distance difference(ΔD_(D)). The relationship between the minimum actuation time interval(T_(MIN)), the nominal distance (D_(N)) 238, the dither distancedifference (ΔD_(D)), and the maximum bottom speed (S_(MAX)) can bedescribed as follows:

$\begin{matrix}{T_{MIN} = \frac{D_{N} - {\Delta \; D_{D}}}{S_{MAX}}} & (3)\end{matrix}$

Solving Expression (3) for S_(MAX) yields:

$\begin{matrix}{S_{MAX} = \frac{D_{N} + {\Delta \; D_{D}}}{T_{MIN}}} & (4)\end{matrix}$

The following example is provided to illustrate the effect of temporallydithering actuations on timing and spacing of the actuations. In theexample of FIG. 2, the minimum actuation time interval (T_(MIN)) is fiveseconds and a nominal distance 238 between nominal actuation points230-1 and 230-2 is 12.5 meters. Without dithering the actuations, inorder to sail the nominal distance (D_(N)) 238 of 12.5 meters with theminimum actuation time interval (T_(MIN)) of five seconds between theactuations, the bottom speed can be 2.5 meters per second (4.9 knots).

Now, assume that the actuations in the example of FIG. 2 are dithered.The dither time corresponding to the nominal actuation point 230-1 isone second and the dither time corresponding to the nominal actuationpoint 230-2 is zero seconds. Although a zero dither time would mean thatthe nominal actuation point 230-2 would be same as the actuation point232-2 (the second dither distance 236 being zero meters), FIG. 2 showsthe actuation point 232-2 being separated from the actuation point 230-2for illustration purposes only. If the bottom speed was 2.5 meters persecond (the speed to travel 12.5 meters in five seconds), the firstdither distance 234 would be 2.5 meters and the second dither distance236 would be zero meters. Based on a bottom speed of 2.5 meters persecond and a dither time difference (ΔT_(D)) of one second, theactuation distance 240 between actuations would be ten meters (thenominal distance 238, 12.5 meters, minus the difference between thefirst dither distance 234, 2.5 meters, and the second dither distance236, zero meters). The actuation distance 240 is represented inExpression (1) by D_(N)−S_(MAX)ΔT_(D). A dither time of one secondfollowed by a dither time of zero seconds would result in the timebetween actuations at the actuation points 232-1 and 232-2 being fourseconds. Thus, instead of traveling 12.5 meters in five seconds, thesource would travel the actuation distance 240 of ten meters in fourseconds. In order to maintain the minimum actuation time interval(T_(MIN)) of five seconds, the bottom speed would have to be reduced tobe slower than 2.5 meters per second (4.9 knots) such that the sourcetravels the actuation distance 240 of approximately ten meters (becausethe bottom speed is reduced the distance traveled within the one seconddifference is reduced as well) in the minimum actuation time interval(T_(MIN)) of five seconds.

Expression (2) can be used to determine the maximum bottom speed(S_(MAX)) such that the source travels the actuation distance 240 of tenmeters, resulting from the dither time difference (ΔT_(D)) being onesecond, in at least the minimum actuation time interval (T_(MIN)) offive seconds. Solving Expression (2) yields a maximum bottom speed(S_(MAX)) of approximately 2.1 meters per second (4.1 knots), which isslower than the 2.5 meters per second (4.9 knots) if the actuationcorresponding to the nominal actuation point 230-2 was not dithered.However, if the dither time difference (ΔT_(D)) is the maximum dithertime difference, the resulting maximum bottom speed (S_(MAX)) could bemaintained during a marine survey for any dither time difference(ΔT_(D)) up to and including the maximum dither time difference, whilemaintaining the minimum actuation time interval. Therefore, the maximumbottom speed could be 2.1 meters per second (4.1 knots) throughout amarine survey and maintain at least five seconds between consecutiveactuations where the nominal distance 238 is 12.5 meters.

Because marine surveys can cover hundreds of kilometers and take monthsto complete, a decrease or increase in the bottom speed caused bydithering of actuations can have significant impacts on the costs of amarine survey. Reducing the bottom speed can increase costs byincreasing a time that it takes to complete the survey. It can bebeneficial to maintain a faster bottom speed throughout the marinesurvey to further reduce the duration and the costs of the marinesurvey.

FIG. 3 illustrates constraining a dither time difference to a thresholddither time difference that is less than a maximum dither timedifference. The marine survey vessel 318 can be analogous to the marinesurvey vessel 218 illustrated in FIG. 2. The nominal actuation points330-1 and 330-2 can be analogous to the nominal actuation points 230-1and 230-2 illustrated in FIG. 2. A source, such as the source 126illustrated in FIG. 1, may or may not be actuated at the nominalactuation points based on the dither time corresponding to eachactuation point as discussed above. The actuation points 332-1 and 332-2can be analogous to the actuation points 232-1 and 232-2 illustrated inFIG. 2. Consecutive actuations of a source can be separated by anactuation time interval. As discussed above, a dither time differencecan be at most the threshold dither time difference. Such a dither timedifference may result in having to reduce the bottom speed in order tomaintain the minimum actuation time interval. However, by constrainingthe dither time difference to a threshold dither time difference that isless than the maximum dither time difference, a faster bottom speed canbe maintained throughout a marine survey.

The first dither distance 334 corresponds to a dither time correspondingto the nominal actuation point 330-1 and a second dither distance 342corresponding to a dither time corresponding to the nominal actuationpoint 330-2. The actuation distance 344 between actuations correspondsto an actuation time interval between actuations. In the example of FIG.3, the minimum actuation time interval is five seconds, the nominaldistance 338 is 12.5 meters, the maximum dither time is one second, andthe maximum dither time difference is one second, as in the example ofFIG. 2.

In contrast to the example of FIG. 2, the dither time difference isconstrained to a threshold dither time difference that is less than themaximum dither time difference. In the example of FIG. 3, the thresholddither time difference is 0.5 seconds. That is, the dither timedifference can be at most 0.5 seconds. Thus, if, for example, the dithertime corresponding to a particular actuation is the maximum dither timeof one second, then the dither time corresponding to a directlysubsequent actuation to the particular actuation can be no less than 0.5seconds. The randomization scheme can control the dither time and, inthis example, ensure that, if the first actuation is dithered by onesecond, the second actuation has to be dithered by at least 0.5 second.Thus, the actuation distance 344 is at least 11.75 meters, assuming abottom speed of 2.5 meters per second, and, following Expression (3), toenforce the threshold dither time difference, the dither timecorresponding to an actuation can be constrained based upon the dithertime corresponding to a directly precedent actuation. For example, ifthe dither time corresponding to the nominal actuation point 230-1 isone second, then a randomization scheme can constrain the dither timecorresponding to the nominal actuation point 230-2 can be at least 0.5seconds. Negative dither times can be constrained as well. For example,if the dither time corresponding to actuation point 230-1 is −0.2seconds, then the dither time corresponding to the actuation point 230-2can be at least −0.7 seconds.

To account for a constraint on the dither time difference (ΔT_(D)), thedither time difference (ΔT_(D)) in Expression (2) can be replaced withthe threshold dither time difference (ΔT_(D) _(th) ), yielding:

$\begin{matrix}{S_{MAX} = \frac{D_{N}}{T_{MIN} + {\Delta \; T_{D_{th}}}}} & (5)\end{matrix}$

Solving Expression (5) where the minimum actuation time interval(T_(MIN)) between consecutive actuations is five seconds, the nominaldistance (D_(N)) 338 is 12.5 meters, and the threshold dither timedifference (ΔT_(D) _(th) ) is 0.5 seconds yields a maximum bottom speedof approximately 2.3 meters per second (4.4 knots). Thus, in contrast tothe example of FIG. 2, by constraining the dither time difference to athreshold dither time difference that is less than the maximum dithertime difference, a faster maximum bottom speed can be maintained duringa marine survey such that there is at least five seconds (the minimumactuation time interval (T_(MIN))) between consecutive actuations at theactuation points 332-1 and 332-2. In the example of FIG. 3, the bottomspeed can be 2.3 meters per second (4.4 knots), which is 0.2 meters persecond (0.3 knots) faster than the 2.1 meters per second (4.1 knots) ofthe example of FIG. 2 and still maintain the minimum actuation timeinterval (T_(MIN)) of five seconds for any dither time difference lessthan the threshold dither time difference (ΔT_(D) _(th) ).

Each actuation of pair of consecutive actuations in a marine survey canhave a corresponding dither time where the dither times are constrainedto a threshold dither time difference. With respect to FIG. 3, thenominal actuation point 330-1 and the actuation point 332-1 cancorrespond to an actuation of a source directly precedent to theactuation corresponding to the nominal actuation point 330-2 and theactuation point 332-2. Although not shown in FIG. 3, the nominalactuation point 330-3 and the actuation point 332-3 can correspond to anactuation of a source directly subsequent to the actuation to which thenominal actuation point 330-2 and the actuation point 332-2 correspond.The dither time for each actuation can be constrained such that areduction of the actuation time interval relative to a directlyprecedent actuation time interval is at most a threshold dither timedifference (ΔT_(D) _(th) ). That is, the dither time for the actuationcorresponding to the nominal actuation point 332-3 can be constrainedsuch that the actuation time interval between the actuations at theactuation points 332-2 and 332-3 is not reduced by more than thethreshold dither time difference (ΔT_(D) _(th) ) relative to theactuation time interval between the actuations at the actuation points332-1 and 332-2. A dither time can be generated corresponding to asubsequent actuation of the source according to a randomization scheme,wherein the subsequent actuation is directly subsequent to the actuationof the source. Another dither time difference between the dither timecorresponding to the subsequent actuation of the source at the actuationpoint 332-3 (not shown in FIG. 3) and the dither time corresponding tothe actuation of the source at the actuation point 332-2 can beconstrained to the threshold dither time difference. The source can beactuated at the actuation point 332-3 after an actuation time intervalfollowing the actuation of the source at the actuation point 332-2,wherein the actuation time interval comprises the nominal time and adither time corresponding to the subsequent actuation of the source atthe actuation point 332-3.

If actuations of a source are spatially dithered, a dither distancedifference can be constrained to a threshold dither distance difference.By constraining a dither distance difference to a threshold ditherdistance difference that is less than a maximum dither distancedifference, a faster bottom speed can be maintained during a survey forany dither distance difference up to and including the threshold ditherdistance difference. For example, the dither distance difference can bethe first dither distance 334 minus the second dither distance 342. Atleast one embodiment in accordance with the present disclosure includesconstraining a difference between a first dither distance correspondingto a first actuation of a source and a second dither distancecorresponding to a second actuation of a source, where the firstactuation is directly precedent to the second actuation. To account fora constraint on the dither distance difference (ΔD_(D)), the ditherdistance difference (ΔD_(D)) in Expression (4) can be replaced with thethreshold dither distance difference (ΔD_(D) _(th) ), yielding:

$\begin{matrix}{S_{MAX} = \frac{D_{N} + {\Delta \; D_{D_{th}}}}{T_{MIN}}} & (6)\end{matrix}$

FIG. 4 illustrates a graph relating minimum actuation time intervals(T_(MIN)), maximum bottom speeds (S_(MAX)), and dither time differences(ΔT_(D)). FIG. 4 assumes a nominal distance of 12.5 meters betweennominal actuation points and a maximum positive dither time of onesecond, such that the dither time varies between zero and a maximumdither time of one second. The line 452 corresponds to a nominal time offive seconds. The curves 450 collectively correspond to various dithertime differences ranging from positive one second to negative onesecond.

The curve 450-1 corresponds to a dither time difference (ΔT_(D)) of onesecond. The curve 450-1 can correspond to the dither time difference(ΔT_(D)) of one second in the example of FIG. 2. A vertical line 454 canbe drawn from the intersection of the line 452 and the curve 450-1 todetermine the maximum speed of a marine survey vessel (S_(MAX)) than canbe maintained throughout the marine survey. FIG. 4 shows that the line454 corresponds to the maximum bottom speed (S_(MAX)) beingapproximately 4.1 knots (approximately 2.1 meters per second), as in theexample of FIG. 2.

The curve 450-2 corresponds to a dither time difference (ΔT_(D)) of 0.5seconds. The curve 450-2 can correspond to the dither time difference(ΔT_(D)) of 0.5 seconds in the example of FIG. 3. A vertical line 456can be drawn from the intersection of the line 452 and the curve 450-2to determine the maximum bottom speed (S_(MAX)) that can be maintainedthroughout the marine survey when the dither time difference (ΔT_(D)) is0.5 seconds. The line 456 corresponds to the maximum bottom speed(S_(MAX)) being approximately 4.4 knots (approximately 2.3 meters persecond), as in the example of FIG. 3.

FIG. 4 highlights a benefit of constraining a dither time difference toa maximum amount of time, such as a threshold dither time difference(ΔT_(D) _(th) ), that is less than the maximum dither time. For example,the dither time difference (ΔT_(D)) can be constrained to 0.5 seconds,which is less than the maximum dither time of one second. Comparing theline 454 corresponding to a dither time difference (ΔT_(D)) of onesecond to the line 456 corresponding to a dither time difference(ΔT_(D)) of 0.5 seconds illustrates that a faster maximum bottom speedcan be maintained according to at least one embodiment of the presentdisclosure where the dither time difference (ΔT_(D)) is constrained toat most 0.5 seconds. Constraining the dither time difference can enablea faster maximum bottom speed while still maintaining the minimumactuation time interval (T_(MIN)) for any dither time difference up toand including the threshold dither time difference.

FIG. 4 demonstrates the flexibility of constraining a dither timedifference to a threshold dither time difference (ΔT_(D) _(th) )according to at least one embodiment of the present disclosure. Forexample, instead of using a minimum actuation time interval (T_(MIN))and a threshold dither time difference (ΔT_(D) _(th) ) to determine acorresponding maximum bottom speed (S_(MAX)), a maximum bottom speed(S_(MAX)) and a threshold dither time difference (ΔT_(D) _(th) ) can beused to determine a corresponding minimum actuation time interval(T_(MIN)). The dither time difference can be constrained to thethreshold dither time difference (ΔT_(D) _(th) ) based upon a particularmaximum bottom speed (S_(MAX)) towing the source to maintain at leastthe minimum actuation time interval (T_(MIN)).

Similarly, a minimum actuation time interval (T_(MIN)) and a maximumbottom speed (S_(MAX)) can be used to determine a correspondingthreshold dither time difference (ΔT_(D) _(th) ). The dither timedifference can be constrained to the threshold dither time difference(ΔT_(D) _(th) ) based on a minimum actuation time interval (T_(MIN)) tomaintain a maximum bottom speed (S_(MAX)) towing the source.

FIG. 5 illustrates a method flow diagram for constraining dithering ofsource actuations. At block 560, the method can include actuate a sourcein sequence, as the source moves through a fluid volume at a bottomspeed, with an actuation distance between each actuation. The actuationdistance can comprise a sum of a nominal distance and a dither distancefor each actuation. The source can be actuated according to an actuationsequence. The method can include near-continuously recording marinesurvey data corresponding to the particular and directly subsequentactuations of the source such that the actuation sequence isuninterrupted.

At block 562, the method can include constraining the dither distancefor each actuation such that a reduction of the actuation distancerelative to a directly precedent actuation distance interval is at mosta threshold dither distance difference corresponding to a maximum bottomspeed. A difference between the dither distance corresponding to aparticular actuation of the source and the dither distance correspondingto a directly subsequent actuation of the source can be constrained tothe threshold dither distance difference.

As used herein, “near-continuous” can include without meaningful breaksin the seismic recording. As would be understood by one of ordinaryskill in the art with the benefit of this disclosure, operationalcircumstances can cause intermittent gaps in records (due to equipmentfailure, etc.), and “near-continuous recording” should be read toinclude records with intermittent or periodic gaps, whether planned orunplanned as well as records without intermittent or periodic gaps, thusincluding “continuous records.” For simplicity, the term“near-continuous” and “near-continuously” will be used herein and do notexclude “continuous” or “continuously.”

Although not shown in FIG. 5, the method can include deblending themarine survey data corresponding to the particular actuation of thesource from the marine survey data corresponding to the directlysubsequent actuation of the source. Deblending the marine survey datacan include distinguishing a signal corresponding to the particularactuation of the source overlapping a signal corresponding to thedirectly subsequent actuation of the source via the dither distance forthe particular actuation and the directly subsequent actuation.Deblending the marine survey data can include distinguishing a signalcorresponding to the particular actuation of a first source of a sourcearray overlapping a signal corresponding to a directly subsequentactuation of a second source of the source array via the dither distancefor the particular actuation and the directly subsequent actuation.

Although not shown in FIG. 5, the method can include recording marinesurvey data corresponding only to the particular actuation of the sourceduring a first portion of the actuation distance, and recording marinesurvey data corresponding to both the particular actuation of the sourceand the directly subsequent actuation of the source during a secondportion of the actuation distance. Although not shown in FIG. 5, themethod can include recording marine survey data corresponding to eachactuation of the source. Recording the marine survey data can includestoring the marine survey data, which was received from a third partythat performed the marine survey.

A system for constraining dithering of source actuations can include adata store, a subsystem, and a number of engines, such as a deblendingengine. The deblending engine can be configured to distinguish marinesurvey data corresponding to a first actuation of a source from marinesurvey data corresponding to a second actuation of a source via a dithertime for each actuation. The subsystem and engines can be incommunication with a data store via a communication link. The system canrepresent program instructions and/or hardware of a machine. As usedherein, an “engine” can include program instructions and/or hardware,but at least includes hardware. Hardware is a physical component of amachine that enables it to perform a function. Examples of hardware caninclude a processing resource, a memory resource, a logic gate, etc.

The number of engines can include a combination of hardware and programinstructions that is configured to perform a number of functionsdescribed herein. The program instructions, such as software, firmware,etc., can be stored in a memory resource such as a machine-readablemedium, etc., as well as hard-wired program such as logic. Hard-wiredprogram instructions can be considered as both program instructions andhardware.

A machine for constraining dithering of source actuations can utilizesoftware, hardware, firmware, and/or logic to perform a number offunctions. The machine can be a combination of hardware and programinstructions configured to perform a number of functions. The hardware,for example, can include a number of processing resources and a numberof memory resources, such as a machine-readable medium or othernon-transitory memory resources. The memory resources can be internaland/or external to the machine, for example, the machine can includeinternal memory resources and have access to external memory resources.The program instructions, such as machine-readable instructions, caninclude instructions stored on the machine-readable medium to implementa particular function, for example, generating a dither timecorresponding to an actuation of the source, constraining a differencebetween the dither time corresponding to the actuation of the source anda previous dither time corresponding to a directly precedent actuationof the source to a threshold dither time difference, and actuating thesource after an actuation time interval following the directly precedentactuation of the source, wherein the actuation time interval comprisesthe dither time corresponding to the actuation of the source. The set ofmachine-readable instructions can be executable by one or more of theprocessing resources. The memory resources can be coupled to the machinein a wired and/or wireless manner. For example, the memory resources canbe an internal memory, a portable memory, a portable disk, or a memoryassociated with another resource, for example, enabling machine-readableinstructions to be transferred and/or executed across a network such asthe Internet. As used herein, a “module” can include programinstructions and/or hardware, but at least includes programinstructions.

Memory resources can be non-transitory and can include volatile and/ornon-volatile memory. Volatile memory can include memory that dependsupon power to store data, such as various types of dynamic random accessmemory among others. Non-volatile memory can include memory that doesnot depend upon power to store data. Examples of non-volatile memory caninclude solid state media such as flash memory, electrically erasableprogrammable read-only memory, phase change random access memory,magnetic memory, optical memory, and a solid state drive, etc., as wellas other types of non-transitory machine-readable media.

The processing resources can be coupled to the memory resources via acommunication path. The communication path can be local or remote to themachine. Examples of a local communication path can include anelectronic bus internal to a machine, where the memory resources are incommunication with the processing resources via the electronic bus.Examples of such electronic buses can include Industry StandardArchitecture, Peripheral Component Interconnect, Advanced TechnologyAttachment, Small Computer System Interface, Universal Serial Bus, amongother types of electronic buses and variants thereof. The communicationpath can be such that the memory resources are remote from theprocessing resources, such as in a network connection between the memoryresources and the processing resources. That is, the communication pathcan be a network connection. Examples of such a network connection caninclude a local area network, wide area network, personal area network,and the Internet, among others.

The machine-readable instructions stored in the memory resources can besegmented into a number of modules that when executed by the processingresources can perform a number of functions. As used herein a moduleincludes a set of instructions included to perform a particular task oraction. The number of modules can be sub-modules of other modules.Furthermore, the number of modules can comprise individual modulesseparate and distinct from one another.

In accordance with a number of embodiments of the present disclosure, ageophysical data product may be produced. The geophysical data productmay include, for example, field data recorded during a survey utilizingthe above-described techniques. Geophysical data may be obtained andstored on a non-transitory, tangible computer-readable medium. In someinstances, once onshore in the United States, geophysical analysis maybe performed on the geophysical data product. In some instances,geophysical analysis may be performed on the geophysical data productoffshore according to techniques described herein or known in the art,and stored on a computer-readable medium, to produce an enhancedgeophysical data product.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

What is claimed is:
 1. A system, comprising: a source; and a controllercoupled to the source, wherein the controller is configured to: actuatethe source in sequence, while the source is moving through a fluidvolume at a bottom speed, with an actuation time interval between eachactuation comprising a sum of a nominal time and a dither time for eachactuation; and constrain the dither time for each actuation such that areduction of the actuation time interval relative to a directlyprecedent actuation time interval is at most a threshold dither timedifference, wherein the threshold dither time difference corresponds toa maximum bottom speed.
 2. The system of claim 1, wherein the dithertime corresponds to a distance offset from each of a plurality ofnominal actuation points, wherein the plurality of nominal actuationpoints are spaced apart at a nominal distance.
 3. The system of claim 1,wherein the source comprises a source array, wherein the source array isconfigured to move through the fluid volume at the bottom speed, andwherein the controller is further configured to: actuate a first sourceof the source array with a first dither time; directly subsequent to theactuation of the first source, actuate a second source of the sourcearray after the actuation time interval comprising the nominal time anda second dither time; and constrain a difference between the firstdither time and the second dither time to at most the threshold dithertime difference.
 4. The system of claim 1, wherein the actuation timeinterval is such that a first signal corresponding to a first actuationof the source overlaps a second signal corresponding to a secondactuation of the source.
 5. The system of claim 1, further comprising: areceiver configured to receive marine survey data from the actuations ofthe source; and a deblending engine configured to distinguish marinesurvey data corresponding to a first actuation of the source from marinesurvey data corresponding to a second actuation of the source via thedither time for the first actuation and the second actuation.
 6. Amethod for performing a marine survey, comprising: actuating a source,as the source moves through a fluid volume at a bottom speed, with anactuation distance between each actuation comprising a sum of a nominaldistance and a dither distance for each actuation; and constraining thedither distance for each actuation such that a reduction of theactuation distance relative to a directly precedent actuation distanceis at most a threshold dither distance difference, wherein the thresholddither distance difference corresponds to a maximum bottom speed.
 7. Themethod of claim 6, wherein constraining the dither distance comprisesconstraining a difference between the dither distance corresponding to aparticular actuation of the source and the dither distance correspondingto a directly subsequent actuation of the source to the threshold ditherdistance difference.
 8. The method of claim 7, wherein actuating thesource comprises actuating the source according to an actuationsequence, and wherein the method further comprises near-continuouslyrecording marine survey data corresponding to the particular anddirectly subsequent actuations of the source such that the actuationsequence is uninterrupted.
 9. The method of claim 7, further comprisingdeblending the marine survey data corresponding to the particularactuation of the source from the marine survey data corresponding to thedirectly subsequent actuation of the source.
 10. The method of claim 9,wherein deblending the marine survey data includes distinguishing asignal corresponding to the particular actuation of the sourceoverlapping a signal corresponding to the directly subsequent actuationof the source via the dither distance for the particular actuation andthe directly subsequent actuation.
 11. The method of claim 7, furthercomprising: recording marine survey data corresponding only to theparticular actuation of the source during a first portion of theactuation distance; and recording marine survey data corresponding toboth the particular actuation of the source and the directly subsequentactuation of the source during a second portion of the actuationdistance.
 12. The method of claim 6, further comprising recording marinesurvey data corresponding to each actuation of the source, whereinrecording the marine survey data comprises storing the marine surveydata, which was received from a third party that performed the marinesurvey.
 13. A non-transitory machine-readable medium storinginstructions executable by a processing resource to: generate a dithertime corresponding to an actuation of a source; constrain a dither timedifference between the dither time corresponding to the actuation of thesource and a previous dither time corresponding to a directly precedentactuation of the source to a threshold dither time difference, whereinthe threshold dither time difference is less than a maximum dither timedifference; and actuate the source after an actuation time intervalfollowing the directly precedent actuation of the source, wherein theactuation time interval comprises the dither time corresponding to theactuation of the source.
 14. The medium of claim 13, wherein theinstructions to constrain the dither time difference compriseinstructions to constrain the dither time difference to the thresholddither time difference based upon a particular bottom speed that isnecessary to maintain at least a minimum actuation time interval,wherein the actuation time interval comprises: a nominal time; and thedither time corresponding to the actuation of the source.
 15. The mediumof claim 14, wherein the nominal time is five seconds, and wherein thethreshold dither time difference is 0.5 seconds and corresponds to theparticular bottom speed being 4.4 knots.
 16. The medium of claim 13,wherein the instructions to constrain the dither time differencecomprise instructions to constrain the dither time difference to thethreshold dither time difference based upon a nominal time that isnecessary to maintain a maximum bottom speed, wherein the actuation timeinterval comprises: the nominal time; and the dither time correspondingto the actuation of the source.
 17. The medium of claim 13, furtherstoring instructions to: generate another dither time corresponding to adirectly subsequent actuation of the source according to a randomizationscheme; and constrain another dither time difference between the otherdither time corresponding to the directly subsequent actuation of thesource and the dither time corresponding to the actuation of the sourceto the threshold dither time difference.
 18. The medium of claim 17,further storing instructions to actuate the source after the actuationtime interval following the actuation of the source, wherein theactuation time interval comprises a nominal time and the other dithertime corresponding to the directly subsequent actuation of the source.19. A method to manufacture a geophysical data product, the methodcomprising: obtaining geophysical data; processing the geophysical datato generate the geophysical data product, wherein processing thegeophysical data comprises: actuating a source in sequence, as thesource moves through a fluid volume at a bottom speed, with an actuationdistance between each actuation comprising a sum of a nominal distanceand a dither distance for each actuation; and constraining the ditherdistance for each actuation such that a reduction of the actuationdistance relative to a directly precedent actuation distance is at mosta threshold dither distance difference, wherein the threshold ditherdistance difference corresponds to a maximum bottom speed; and recordingthe geophysical data product on a non-transitory machine-readablemedium.
 20. The method of claim 19, wherein processing the geophysicaldata comprises processing the geophysical data offshore or onshore.